In computer systems, and particularly those which incorporate multiple processors, it is neccesary to have a system for determining priority between commands or instructions which cannot be executed at the same time. Most prior art priority systems are fixed in whole or in part and do not allow the flexibility that is desirable.
What is needed is some type of priority system for use when one or more requests is submitted to a serial path within a processing structure. The priority system should, for all practical purposes, select the request in a known order that will be desired for the system. If the best priority order is unknown at the time the machine is built or if the best priority order changes from operation to operation, some programmability will be needed.